Level I: North America has been divided into 15 broad, level I ecological regions. These highlight major ecological areas and provide the broad backdrop to the ecological mosaic of the continent, putting it in context at global or intercontinental scales.

Brief narrative descriptions of each level I region can be found in Section III of the Commission for Environmental Cooperation's publication "Ecological Regions of North America--Toward a Common Perspective" (CEC 1997). These descriptionseach of which is divided into sections describing the physical setting, biological setting and human activities thereinprovide an overview of the principal attributes of each region. The intent is to provide a sense of the ecological diversity, the human interactions taking place and how each region differs from adjacent ones.

Level II: The 50 level II ecological regions that have been delineated are intended to provide a more detailed description of the large ecological areas nested within the level I regions. For example, the Tropical Wet Forests of level I is the region covering coastal portions of the United States and Mexico, and is composed of six level II regions. Level II ecological regions are useful for national and subcontinental overviews of ecological patterns. Three level I regions (Hudson Plains, Marine West Coast Forest, and Mediterranean California) have no subdivisions at level II. The Temperate Sierras, Tropical Dry Forests, and Tropical Wet Forests level I regions, on the other hand, each have six level II subdivisions.

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